var stream = require("stream");
var Readable = stream.Readable;
var rs = new Readable();
var c = 'a'.charCodeAt(0);
rs.read = function() {
    if(c < 'z'.charCodeAt(0)) {
        rs.push(String.fromCharCode(c++));
    }else{
        rs.push(null);
    }
}
// 将可读流输出一个可写流
rs.pipe(process.stdout);